Of the three xylene isomers, paraxylene is the most commercially valuable. However, due to the similarity of their boiling points, adsorption is a commonly used method to separate paraxylene from the other xylene isomers, in which an adsorbent solid which preferentially adsorbs paraxylene over metaxylene and orthoxylene is used.
A commercial embodiment of a simulated moving-bed adsorption apparatus is used in the well-known Parex™ Process, which is used to separate C8 aromatic isomers and provide a more highly pure paraxylene (PX) from a less highly pure mixture. See by way of example U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717. Other embodiments involving a simulated moving-bed adsorption apparatus include ELUXYL™, available from Axens, and AROMAX™, available from Toray.
In a Parex™ unit, the locations of liquid input and output are moved by a fluid directing device described herein as a rotary valve device. This device may comprise one or more rotary valves, as well as various control and accessory means, such as inlet lines, outlets lines and valves associated therewith. The rotary valve device works in conjunction with conduits located between the adsorbent beds. The rotary valve device accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific conduits in fluid communication with particular adsorbent beds. After a specified time period, called the step time, the rotary valve device advances one index and redirects the liquid inputs and outputs to the conduit immediately adjacent and downstream of the previously used conduits. Each advancement of the rotary valve device to a new position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time or step interval is uniform for each valve step in a valve cycle, and may be from about 30 seconds to 4 minutes.
An example of a commercial simulated moving-bed adsorption apparatus contains 24 adsorbent beds and 24 conduits individually connected to a bed and providing fluid communication with the rotary valve device. The conduits of the adsorption apparatus may function, over time, as at least two liquid input lines (e.g., a feed input line and a desorbent input line) and two liquid output lines (e.g., an extract withdrawal line and a reformate withdrawal line).
A system employing a simulated countercurrent flow process such as described in U.S. Pat. Nos. 3,201,491; 3,761,533; 4,029,717; and 8,529,757 are shown in FIG. 1, along with several modifications. The diagram in FIG. 1 will be understood by those of skill in the art to depict a simulated moving-bed process. Desorbent is introduced through conduit 100. Liquid withdrawal stream leaves the apparatus through conduit 101. Extract (containing the desired product) leaves the apparatus via conduit 102. Raffinate leaves the apparatus through conduit 110. A C8 aromatic feed, which comprises 15 to 30 volume percent paraxylene, is added to the apparatus through conduit 107. Optionally, a C8 aromatic mixture, which comprises 75 to 98 volume percent paraxylene, is added as an additional feed through conduit 108.
Not shown in the drawing, but as would be recognized by one of skill in the art in possession of the disclosure of U.S. Pat. No. 8,529,757, is one or more distillation towers and attendant pumps and conduits, which may be utilized to purify the liquid withdrawal stream leaving the above-described apparatus via conduit 101.
Continuing with the description of FIG. 1, the arrow 112 represents the simulated movement of beds upward through apparatus 120 containing plural adsorption bed chambers A1 through An+j. Arrow 111 represents the countercurrent flow of circulating bulk fluid to the adsorbent beds. In operation, the adsorbent does not flow, but the various inlet and outlet streams, such as feed, product and flush streams, cycle through the adsorbent bed chambers, represented by chambers A1 through An+j, in a direction, which is countercurrent to the simulated movement of adsorbent beds and cocurrent to the direction of the circulating bulk fluid. This simulates the movement of the adsorbent beds A1 through An+j. Theoretically, there may be any number of adsorbent beds, thus n>2 and n+j is the maximum number of adsorbent beds. However, from a practical standpoint the number of bed lines is limited by design considerations and other factors. It will be understood that n and j are positive integers and that in an example of a commercial embodiment the total number of adsorbent beds is 24, and thus n+j typically may be 24. Certain adsorbent beds, i.e., beds between A2 and An, beds An+3, An+5, An+6, and An+10 through An+j−1 are not depicted in FIG. 1, for convenience of view.
In the unit shown in FIG. 1, xylene and ethylbenzene molecules from feed 107 are adsorbed in bed An+9. As the adsorbent in bed An+9 becomes saturated with C8 aromatics, a portion of the C8 aromatics in the feed flow along with circulating bulk fluid and flow into at least one bed, such as An+10 (not shown in FIG. 1), below bed An+9. According to a predetermined cycle time, the flow of feed, along with the flows of other inlet and outlet streams, is shifted to one adsorbent bed below. In FIG. 1 the bed below An+9 would be bed An+10 (not shown in FIG. 1). The direction of the shifting of feed and other streams to and from the adsorbent apparatus is the same as the direction of the flow of the circulating bulk fluid through the apparatus. This shifting of streams results in adsorbed C8 aromatics being moved (in a simulated manner) to beds above the bed to which feed is being introduced at any given time.
The feed which is introduced through conduit 107 may comprise equilibrium xylenes (such as from a powerformer, isomerization unit or transalkylation unit). Such equilibrium xylenes may comprise about 21-24 wt % paraxylene (PX). A portion of the feed introduced through conduit 107, may also comprise enhanced paraxylene, for example, from a selective toluene disproportionation unit (0 unit). This enhanced paraxylene may comprise, for example, about 85-90 wt % PX. In one embodiment, the feed to introduce through conduit 107 is free of enhanced paraxylene from a toluene disproportionation process.
The paraxylene is desorbed from adsorbent in the beds by desorbent, which is introduced into bed A1 of the adsorption apparatus through conduit 100. The desorbent displaces paraxylene from the adsorbent. The desorbent also has a different boiling point than the C8 aromatics and is easily separated from C8 aromatics in a distillation process. Examples of desorbents include paradiethylbenzene (pDEB), toluene (TOL), or a mixture thereof. The stream, which is introduced into the apparatus through conduit 100, may, optionally, also comprise a diluent, such as a non-aromatic (NA) hydrocarbon, which has less binding affinity to the adsorbent than any of the C8 aromatics. However, such diluents take up volume in the apparatus and are not necessary. Accordingly the stream, which is introduced into the apparatus through conduit 100, is preferably free of such diluent.
An extract stream is withdrawn from bed An through conduit 102. The extract stream comprises a mixture of the purified paraxylene and the desorbent. As shown in FIG. 1, the withdrawal point of the extract stream though conduit 102 is between the point of introduction of the feed through conduit 107 and the point of introduction of the desorbent through conduit 100. A raffinate stream is withdrawn from bed An+j through conduit 110. The raffinate stream comprises paraxylene-depleted C8 aromatics and desorbent.
FIG. 1 depicts a simplified simulated moving-bed apparatus, wherein countercurrent “movement” of the solids in beds A1 through An+j relative to the fluid streams may be simulated by the use of a rotary valve, which is not shown in the FIG. 1. As the valve rotates, the zones previously discussed move through the column in a stepwise sequence due to the change in the stream flows through the valve. In certain embodiments, a rotary valve, as described in U.S. Pat. No. 3,205,166, may be used. In this arrangement, each fluid communication conduit connected to the chamber may serve a different function with each step rotation of the rotary valve.
In standard simulated moving-bed separation processes, the flow rate of streams into and out of the simulated moving-bed are held constant during the step time. However, modulation of flow during the step time has been found to enhance separation in certain instances involving simulated moving-bed separation of fructose and glucose or separation of 1,1′-bi-2-naphthol enatiomers. The enhanced separation may result in greater purity of product streams or less desorbent use. This process for modulating flow rates during a step time has been referred to as a PowerFeed process. Examples of PowerFeed processes are described in an article by Kawajiri et al., “Optimization strategies for simulated moving bed and PowerFeed processes”, AIChE J. Vol. 52 (2006) B, pp. 1343-1350, and in an article by Zhang et al., “PowerFeed operation of simulated moving bed units: changing flow-rates during the switching interval”, Journal of Chromatography A. 1006, pp. 87-99, 2003, Elsevier B.V.
There is an ongoing need to further improve the simulated moving-bed adsorption process, maximize the purity of product streams and make the process more efficient.